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Why exactly are flaps needed when landing? What's the difference between, say, landing with 20 flaps and landing with no flaps at all?

This is a question for the flight school forum.


At low speeds, flaps induce drag, slowing down the aircraft.  At high throttle settings, flaps increase rate of climb, allowing shorter takeoff roles.  My reccomendation is that you go through the lessons to learn about all of the different systems.

While flaps do certainly create drag, that's not the most crucial reason they're needed for landing, since there are ways other than drag to slow down an aircraft.  The primary reason you'd want to use flaps while landing is because extending the flaps lowers the minimum speed before you lose lift.  So say you need to keep your speed above 100 while landing without flaps, if you put the flaps down, that minimum speed might be 90.  In short, they generate more lift.

Why aren't the flaps always extended if they create more lift?  Well that's when drag becomes an issue.  After a certain point, the flaps generate too much drag to justify their usefulness.  Any benefits they gave in terms of increased lift would be overridden by decreased speed.

EDIT: I should also say that the drag flaps generate does come in quite useful for allowing steeper landings that normal.  But still, it's only a secondary benefit.  The primary function of flaps is the generation of lift.

Flaps come in three major categories. Simple (the rear part of the wing drops down), Split (the underside at the back of the wing drops down), and Fowler (the underside at the back of the wing drops down and backwards). They are used primarily for taking off and landing. Some aircraft can use them for better slow-speed manoeuvring.  The use of flaps is directly related to the type of flap and the type of installation on each aircraft.  Some flaps produce nearly all drag and are only used for landing e.g. the Supermarine Seafire III which had only two settings closed or all the way open (with brick-like aerodynamics).  Others have settings that produce more lift and are very valuable in a pinch by lowering the stall speed and increasing your ability to turn at slow speeds (the Fowler)

[quote]While flaps do certainly create drag, that's not the most crucial reason they're needed for landing, since there are ways other than drag to slow down an aircraft.

Simply, "to increase the angle of approach without increasing airspeed" - as quoted from something I remember reading a lot during flight training.

Everyone's missing the two most  important facts about flaps:

1) Flaps are NOT speed brakes, although yes, of course they will create drag.
But their purpose is to increase the effective camber (curvature between leading and trailing edges) of the wing, thus producing more lift at lower speeds. That's it. When you consider this fact alone, the various situations where various degrees of flap are used make more sense. An excellent analogy: birds' wings. Watch any bird taking off or landing: they don't just flap faster and change the average angle of attack of their wings- they also change the camber of their wings in slow flight or for short landings or takeoffs.

2) Flaps are NOT needed for landing, even in heavy, fast aircraft that come with manuals saying flaps must be used. Flaps just make landings easier and safer, allowing a fairly steep approach angle without a high sink rate or forward rate of speed, which is always helpful.

It's all covered pretty well here. Nothing to add, maybe just to re-state that there are different types of flaps, and that they can have different primary and secondary uses, depending on the plane.. AND the situation.

I'll stick with a plane I know well; a Cessna 172... and one of the original questions:

also if you put flaps down full you an make a 737 do a endo

I'm not kidding i've done it

2) Flaps are NOT needed for landing, even in heavy, fast aircraft that come with manuals saying flaps must be used. Flaps just make landings easier and safer, allowing a fairly steep approach angle without a high sink rate or forward rate of speed, which is always helpful.

The other thing to say on that point is that flaps allow us to get away with runways which are only a couple of miles long!

2) Flaps are NOT needed for landing, even in heavy, fast aircraft that come with manuals saying flaps must be used. Flaps just make landings easier and safer, allowing a fairly steep approach angle without a high sink rate or forward rate of speed, which is always helpful.

In the world of airport firefighting (my career field by chance), a heavy fast aircraft landing with no flaps is called an EMERGENCY.  

2) Flaps are NOT needed for landing, even in heavy, fast aircraft that come with manuals saying flaps must be used. Flaps just make landings easier and safer, allowing a fairly steep approach angle without a high sink rate or forward rate of speed, which is always helpful.

In the world of airport firefighting (my career field by chance), a heavy fast aircraft landing with no flaps is called an EMERGENCY.  

Quite. In the type I'll soon be flying a flapless landing is not something to be taken lightly - they are only practiced on touch and goes and overshoots. Landing speed it in the region of 25kts faster and it takes up to a third more runway. Not too mention you'll quite possibly be changing all the tyres and brakes afterwards too (and potentially blocking the runway).

I should have added "provided there is enough runway" to my earlier comment about flaps not being necessary...

It's true- even an old C172 will seem to float forever with no flaps, even with a good headwind.
But slipping the heck out of it on final helps...

[quote]Everyone's missing the two most